It is proposed that an infinite sequence of linear polymer homologues be described as an open thermodynamic system whose equilibrium state is characterized by an additional parameter, i.e. by the length of macromolecules. This idea has led to the derivation of basic equations for the energy of a system: the repeat-units of chain molecules and the endgroups figure as independent components, like other substances present in the system. An analysis of the phase equilibria led to the derivation of an analogue of the known generalized differential van der Waals equation.